Performance and suitability assessment of controlled islanding methods for online WAMPAC application
Introduction
A Wide-Area Monitoring System (WAMS) can be used to enable advanced protection and control strategies that can counteract the propagation of large disturbances and limit power system blackouts [1], [2]. An example of these advanced strategies is Controlled Islanding, which entails splitting a power system into a set of stable, electrically isolated islands and is an efficient corrective measure that can help power systems survive extreme contingencies, such as: un-damped oscillations, voltage collapse and cascading trips. The islands created are based on an islanding solution and different methods have been proposed for finding possible islanding solutions, i.e. which lines should be opened to split the power system into islands.
Many methods, e.g. Ordered Binary Decision Diagram (OBDD)-based methods, have been proposed to find the set of transmission lines that should be disconnected to create islands with good power-balance and coherent generators [3]. However, the computational efficiency of these methods may not allow them to satisfy the requirement of producing an islanding solution online, due to combinatorial explosion of the searching space. In an attempt to satisfy the requirement for an online solution, heuristic methods [4], [5], [6], [7], the Weak Connection method [8], [9] and the Spectral Clustering method [10], [11] have been proposed.
A significant potential cause of a wide area blackout is the unstable power swings that will develop as groups of generators within the system begin to lose synchronism with one another. These generator groups are referred to as coherent generator groups, as the generators within one group remain coherent with one another but cannot remain coherent with the generators in any other coherent group. As such, many islanding methods are designed to identify these coherent groups and create an islanding solution that will separate them from one another; consequently eliminating the oscillations that were a threatening to cause a wide area blackout.
All of the methods considered here include the separation of the coherent groups as a key part of the islanding solutions they create. However, despite having the same intent, each method will produce a different solution for the same network; these different solutions will also have different strengths and weaknesses. These differences, and their consequences, have neither been thoroughly addressed nor analyzed in the literature. Without a proper understanding of the differences inherent to each of these methods it will not be possible to make a reasoned choice between them when developing a controlled islanding scheme. Therefore, this paper seeks to aid in the development of such an understanding in the hope that it will serve as a sound basis for the development of feasible controlled islanding methods for future networks.
In this paper, the OBDD, Weak Connection and Spectral Clustering based islanding methods are assessed and contrasted in terms of their objective function, graph model, solving algorithm and time complexity. These methods are selected for this assessment because they represent a broad range of the methods that are currently available in the field of controlled islanding. They include methods that use a node weighted graph and methods that use edge weighted graph models that can be either directed or undirected (Section ‘Graph models of controlled islanding’). The islanding problem posed by these graph models can then be solved for one of the three commonly used objective functions, namely: minimum power imbalance, minimum dynamic coupling and minimum power-flow disruption. The optimum solution to this problem can be found using either an exhaustive search that returns every possible solution or an optimization algorithm that returns a single optimal, or near optimal, solution.
The main body of the paper is organized as follows: Section ‘Graph models and controlled islanding methods’ introduces the graph models used for controlled islanding and the three methods being assessed in this paper. Section ‘Theoretical assessment of the three methods’ presents a theoretical assessment and discussion of these methods. In Section ‘Simulation assessment of the three methods’, a case of the IEEE 118-bus system is presented to explore the strengths and weaknesses of each method. In Section ‘Criteria for selecting a suitable islanding method as part of a feasible WAMPAC application’, the criteria for selecting and islanding method for practical use and the feasibility of controlled islanding methods for use as part of a WAMPAC (Wide Area Monitoring, Protection and Control) application are discussed. Section ‘Conclusion’ concludes the paper.
Section snippets
Graph models and controlled islanding methods
This section outlines the graph models used for controlled islanding and then describes the OBDD, Weak Connection and Spectral Clustering based methods for determining controlled islanding solutions.
Theoretical assessment of the three methods
The three methods introduced in Section ‘Graph models and controlled islanding methods’ convert the controlled islanding problem into a graph-cut problem and then a combinational optimization problem, albeit using different graph models and objective functions. This section assesses these three methods in terms of the objective function, graph model, time complexity and solution algorithm.
Simulation assessment of the three methods
The differences shown in Table 1 will cause each method to perform differently in terms of solution accuracy, solution distribution and computational requirements. This section assesses the performance of the three different methods for these criteria. Time-domain simulations are also used to explore the effects of implementing the different islanding solutions. This assessment is performed using the IEEE 118-bus test system. The generator data for the classical generator models used in this
Criteria for selecting a suitable islanding method as part of a feasible WAMPAC application
In this section, the feasibility of controlled islanding as a WAMPAC application and the criteria for selecting an islanding method to be used by such an application are discussed.
Conclusion
This paper critically assesses the performance of three controlled islanding methods: OBDD, Weak Connection and Spectral Clustering. An assessment of this nature allows a deeper understanding of how the simplified islanding methods that have been proposed to solve the complex controlled islanding problem influences the islanding solution produced. The differences between the solutions found by a range of islanding methods to the same islanding problem have not been previously addressed in the
Acknowledgement
This work was supported by the NFSC under Grant 51477093, Taishan Scholars Program of Shandong Province, China, and the Yong Scholars Program of Shandong University.
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